This invention relates to X-ray tubes. More specifically, this invention relates to a noise reduction system or technique for an X-ray tube.
The use of X-ray tubes is quite common. Such X-ray tubes have many uses, one of the most common being as a medical tool. The X-ray system allows a doctor to view features within a patient.
Unfortunately, an X-ray tube operating at steady state condition generates significant noise. The sound pressure level may vary from one X-ray tube design tube to another, but is often about 78 to 80 dB for X-ray systems commonly used in the medical environment. At that level of sound, communication between a patient and doctor is difficult.
An X-ray tube has a vacuum tube mounted in it. The vacuum tube includes an electron emitter which supplies electrons to a target. The target is rotating so as to prevent it from burning out from the electrons supplied to it. The target is used to generate the X-ray radiation. The rotation of the target generates vibration which is transmitted through to the casing by various structural paths, the casing surrounding the vacuum tube and having oil disposed between the vacuum tube and the casing. The oil is used to dissipate heat generated by the vacuum tube and to serve as a dielectric. When the vibration source energy is received by the casing, the casing in turn radiates the sound. Unfortunately, the casing is very thick and has non-uniform spatial distribution so that the chances to reduce the sound radiation from the casing are quite restricted. Additionally, the strong coupling between various of the X-ray tube components hinders any attempt to prevent transmission of vibration energy through to the casing. For example, the oil for dielectric and cooling purposes has little or no attenuation of the vibration energy because the oil is basically incompressable. Also, the vacuum glass surrounding the vacuum tube is a lightweight material so that even a small amount of transmitted energy causes significant vibration of the glass.
As mentioned, the rotation of the target and associated components, more specifically bearings at opposite ends of a rotor shaft used to rotate the target, are responsible for the bearing vibration which leads to the noise. Although one might limit the noise by lowering the rotation speed, this would make it necessary to lower the power of the X-ray and/or lead to damage or shortened life expectancy of the rotating target. Therefore, the lowering of the rotation frequency may not be suitable unless one is willing to sacrifice the image quality.